In the aluminum smelting industry, it is of common practice to produce “green” anodes by mixing petroleum coke, coal tar pitch and usually also buts of anodes recycled from the electrolysis cells. Such a mixing is carried out in pieces of equipment called “anode paste mixers”. It is also of common practice to subject the green anodes to baking in an open or closed furnace.
During the production of the green anodes, fumes are emitted, which contain substantial amounts of heavy gaseous and liquid hydrocarbon vapors from the hot coal tar pitch together with carbon particulates from the petroleum coke and some fluorides from the anode buts.
To control such an emission of fumes, a method is presently being used, which comprises injecting coke dust in the fumes, collecting ducts to absorb the hydrocarbon liquids and vapors contained in the fumes, then filtrating the fumes by means of a bag filter to collect the coke dust, recycling the so collected coke dust to the anode paste mixers, and discharging the remaining fumes to the atmosphere. In this method, any gaseous fluorides present in the fumes are also discharged. Moreover, the coke dust handling and injection system together with the equipment used for recycling of the coke dust to the anode paste mixers, are costly in capital and maintenance.
During the baking of the anodes, the fumes formed in the anode baking furnaces are discharged from said furnaces at temperatures generally above the saturation temperature of the tars which are largely in the form of vapors. These fumes also contain appreciable amounts of fluoride vapors.
To control this other emission of fumes, another method is presently being used, which comprises the steps of dry scrubbing the fumes with powered alumina and recycling said alumina to the electrolysis cells. This is very effective to remove the fluorides, but the dry scrubbing step permits to remove only the hydrocarbons that are in a condensed state. Thus, cooling of the fumes by indirect exchange or by water injection before their filtration is necessary to protect the filter bags of synthetic cloth used for the scrubbing from over heating. Such favors the condensation of hydrocarbon vapors and their collection in the filter.
As it may be appreciated, the current method used for treating the fumes emitted by the anode baking furnaces is effective for removing the fluorides and the heavier fractions of the organic components contained in the fumes, but it does not prevent escape of much of the lighter fractions which contain toxic polyaromatic hydrocarbons (PAH).